1. Field of the Invention
The invention relates generally to the field of energy generation, energy supply and energy management and , more particularly, to a method for operating an entire energy supply network, where energy generation is decentralized and the energy supply network has three supply levels, where energy is generated at each supply level and fed into the respective supply level.
2. Description of the Related Art
During recent years, energy supply networks for the transmission and distribution of electrical energy have undergone considerable change with respect to their composition. Here, an energy supply network is normally understood to mean a network of electrical lines in which physical processes can be described by Kirchhoff's rules. In an energy supply network of traditional composition, electrical energy is transmitted from a small number of centralized large generators (e.g., conventional power plant operators, etc.) to a multiplicity of end users or subscribers (e.g., firms or households) who obtain energy from the energy supply network. Therefore, a transmission direction for the electrical energy is essentially predetermined, i.e., from the large generators (as source) to the individual end users (as destination).
For the purpose of transporting the energy, provision is usually made for three supply levels in an energy supply network, the supply levels essentially being networks or network levels having different specified voltage ranges. In this case, the supply levels are organized according to the voltage range that is used for the transmission of electrical energy, and according to the corresponding distribution function in the energy supply network. Therefore, an energy supply network usually has a high-voltage or transmission level, a medium-voltage or distribution level and low-voltage or final-distribution level.
In an energy supply network of traditional composition, electrical energy that is generated by large generators (as source), such as large hydroelectric power plants, thermal power plants or large wind farms, is fed into the high-voltage level or transmission level and then routed onwards via, e.g., power transformers to the distribution or medium-voltage level. In this case, the transmission level is operated in an extra-high-voltage range or high-voltage range, e.g., in Europe in a voltage range of 60 kV (kilovolts) to 380 kV and higher, and is usually managed centrally by a system operator. At the distribution or medium-voltage level, the electrical energy is usually distributed to regional points, e.g., to transformer stations and/or large entities (consumers), such as factories or hospitals. The distribution level is normally supplied with electrical energy from the hierarchically higher supply level, the transmission or high-voltage level, via substations in a demand-oriented or usage-oriented manner, for example, and is operated in a medium-voltage range (e.g., 1 kV to 60 kV). The final distribution of the electrical energy, in particular to the small end users such as, households or smaller industrial firms, usually takes place at the low-voltage or final-distribution level, a voltage range between approximately 230/400 V or up to 1000 V being used in central Europe. This means that the energy is then transformed from the distribution level to the voltage range of the final-distribution level, and the subscribers or end users connected to the final-distribution level are then supplied with energy. The distribution level or medium-voltage level and the final-distribution level or low-voltage level in an energy supply network of traditional composition are usually jointly managed by, e.g., regional system operators (e.g., regional energy suppliers or regional network operators) in a demand-oriented manner. In other words, electrical energy is obtained as required from the hierarchically higher supply level, in particular the high-voltage or transmission level.
An energy supply network of traditional composition, like many of the energy supply networks in operation today, therefore has a central or hierarchical structure, particularly with respect to operation and management. Here, the required energy is fed in at the highest supply level (the transmission or high-voltage level) and is routed onwards from there to the subsidiary supply levels, i.e., the distribution and final-distribution levels. This means that the energy is always routed from the highest supply level and from one or more central large generators (e.g., hydraulic power plants or thermal power plants) under the control of, e.g., a central system operator to the subsidiary supply levels and the consumers attached thereto, where the subsidiary supply levels (i.e., medium-voltage and low-voltage levels) are managed jointly by one or more system operators. Automated regulation is normally provided at the highest supply level or the transmission level, and the entire energy supply network is controlled by the use or demand at the two lower supply levels (i.e., the distribution level and the final-distribution level), where the energy generation or feed-in at the highest supply level is adapted according to the predicted and/or actual energy demand from the consumer.
Recently, however, attempts to liberalize the energy markets and intensified use of renewable energy resources (e.g., hydraulic power, wind energy or solar radiation) have resulted in the emergence of a multiplicity of smaller energy suppliers that are distributed locally in the energy supply network, e.g., small wind energy installations, small hydraulic power plants or solar or photovoltaic arrays. As a consequence, electrical energy provided by these decentralized energy generators has been fed into the energy supply network at medium-voltage and/or low-voltage level for a number of years. However, this has produced a radical change in the previously normal transmission direction, and a multiplicity of existing decentralized energy generators represents a new challenge for the operation and control of energy supply networks, primarily because many of the central regulating systems previously used in traditional energy supply networks are no longer suitable for operating the energy supply network when decentralized energy generation is included.
Whereas supply difficulties arise due to temporally varying demand for electrical energy by the end users in a traditional energy supply network, an energy supply network that includes decentralized energy generation has further problems in terms of the widely varying availability of electrical energy from decentralized energy generation. This is dependent on the presence of uncontrollable primary sources (e.g., wind or solar radiation), for example, and therefore offers significantly less scope for planning than energy generation using conventional power plants. This direct dependence of the decentralized energy generation on, e.g., current regional weather conditions can result in significant fluctuations in an energy amount that is fed into an energy supply network. As a result of the varying feed-in, significant fluctuations are therefore produced at the supply level, e.g., at the low-voltage level in the case of photovoltaic systems and, e.g., at the low-voltage and/or medium-voltage level in the case of wind power, and these fluctuations must then be monitored at the supply level that is hierarchically higher in each case.
In technical terms, the varying energy feed-in can result in, e.g., sudden excesses or sudden breakdowns of the voltage level in at least parts of the respective supply level, and lead to a varying quality of the energy supply. If more energy is generated than consumed at one of the lower supply levels, there is also a reversal in the flow direction of the energy, and energy from a lower supply level is fed back to a higher supply level. This can result in significant problems with respect to the operational safety and reliability of the energy supply network.
In order to allow simpler control of an entire energy supply network that includes decentralized energy generation, greater reliability of the energy supply, and easier marketing of locally generated energy, the energy amounts provided by decentralized energy generators, said amounts being generally small in comparison with large generators are, e.g., combined to form a virtual power plant. In this case, a virtual power plant (VPP) is a name given to such a combination of a plurality of smaller, decentralized energy generators, such as photovoltaic installations, or small wind energy installations. A control characteristic or overall schedule for a virtual power plant can then be negotiated with the respective network operator, for example, based on predictions relating to future energy demand on the supply side. It is then possible to specify individual schedules, for example, which are applied to regulate the operation of the individual energy generators that are combined in the virtual power plant.
In order to define the overall schedule and the individual schedules, and to control the decentralized energy generators, it is however also necessary to provide a corresponding centralized control and information links to the decentralized energy generators to allow a corresponding information exchange. The European Union supports projects for standardization of control and communication for virtual power plants, e.g. FENIX: Flexible Electricity Network to Integrate the expected ‘energy evolution’, DISPOWER: Distributed Generation with High
Penetration of Renewable Energy Sources, etc. These are intended to allow, e.g., Internet-based control of a virtual power plant and automatic trading of electrical energy. However, virtual power plants have a significant disadvantage in that their centralized control, and the permanent information linking of the decentralized energy generators for this control, involves considerable effort and cost. In particular, each decentralized energy generator must have a continuous information connection to the virtual power plant, and this involves considerable effort and cost for a small operator of a private photovoltaic installation, for example.